Method of performing link adaptation procedure

ABSTRACT

A method of performing link adaptation in a wireless local area network system by a mobile station The mobile station receives a first physical layer protocol data unit (PPDU) transmitted by an access point (AP) via multi-user multiple input/multiple output (MU MIMO) transmission to a plurality of mobile stations. The first PPDU includes a first feedback sequence identifier that includes a value identifying the plurality of mobile stations. When the mobile station is one of the plurality of mobile stations identified by the value included by the first feedback sequence identifier, a modulation and coding scheme (MCS) is estimated based on the first PPDU. The mobile station transmits a second PPDU to the AP. The second PPDU includes channel information about the estimated MCS and a second feedback sequence identifier. The second feedback sequence identifier includes the value included by the first feedback sequence identifier.

This application is a Continuation of co-pending U.S. patent applicationSer. No. 14/643,922 filed Mar. 10, 2015, which is a continuation of U.S.patent application Ser. No. 14/109,713 filed Dec. 17, 2013 (now U.S.Pat. No. 9,019,862), which is a continuation of U.S. patent applicationSer. No. 13/320,722 filed Nov. 15, 2011 (now U.S. Pat. No. 8,634,395),which is the national phase of PCT International Application No.PCT/KR2009/006776 filed Nov. 18, 2009, which claims priority under 35U.S.C. §119(e) to U.S. Provisional Application No. 61/219,386 filed Jun.23, 2009, and under 35 U.S.C. §119(a) claims priority to KoreanApplication No. 10-2009-0082774 filed Sep. 3, 2009. The contents of allof these applications are hereby incorporated by reference as fully setforth herein in their entirety. This application is also related to U.S.patent application Ser. No. 14/109,646 filed Dec. 17, 2013 (now U.S.Pat. No. 8,948,152), which is a continuation of U.S. application Ser.No. 13/320,722.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wireless local area network (WLAN),and more particularly, to a method of allocating a radio resource in avery high throughput (VHT) WLAN system.

2. Discussion of the Related Art

With the advancement of information communication technologies, variouswireless communication technologies have recently been developed. Amongthe wireless communication technologies, a wireless local area network(WLAN) is a technology whereby Internet access is possible in a wirelessfashion in homes or businesses or in a region providing a specificservice by using a portable terminal such as a personal digitalassistant (PDA), a laptop computer, a portable multimedia player (PMP),etc.

Ever since the institute of electrical and electronics engineers (IEEE)802, i.e., a standardization organization for WLAN technologies, wasestablished in February 1980, many standardization works have beenconducted.

In the initial WLAN technology, a frequency of 2.4 GHz was usedaccording to the IEEE 802.11 to support a data rate of 1 to 2 Mbps byusing frequency hopping, spread spectrum, infrared communication, etc.Recently, the WLAN technology can support a data rate of up to 54 Mbpsby using orthogonal frequency division multiplex (OFDM). In addition,the IEEE 802.11 is developing or commercializing standards of varioustechnologies such as quality of service (QoS) improvement, access point(AP) protocol compatibility, security enhancement, radio resourcemeasurement, wireless access in vehicular environments, fast roaming,mesh networks, inter-working with external networks, wireless networkmanagement, etc.

In the IEEE 802.11, the IEEE 802.11b supports a data rate of up to 11Mbps by using a frequency band of 2.4 GHz. The IEEE 802.11acommercialized after the IEEE 802.11b uses a frequency band of 5 GHzinstead of the frequency band of 2.4 GHz and thus significantly reducesinfluence of interference in comparison with the very congestedfrequency band of 2.4 GHz. In addition, the IEEE 802.11a has improvedthe data rate to up to 54 Mbps by using the OFDM technology.Disadvantageously, however, the IEEE 802.11a has a shorter communicationdistance than the IEEE 802.11b. Similarly to the IEEE 802.11b, the IEEE802.11g implements the data rate of up to 54 Mbps by using the frequencyband of 2.4 GHz. Due to its backward compatibility, the IEEE 802.11g isdrawing attention, and is advantageous over the IEEE 802.11a in terms ofthe communication distance.

The IEEE 802.11n is a technical standard relatively recently introducedto overcome a limited data rate which has been considered as a drawbackin the WLAN. The IEEE 802.11n is devised to increase network speed andreliability and to extend an operational distance of a wireless network.

More specifically, the IEEE 802.11n supports a high throughput (HT),i.e., a data processing speed of up to 540 Mbps at a frequency band of 5GHz, and is based on a multiple input and multiple output (MIMO)technique which uses multiple antennas in both a transmitter and areceiver to minimize a transmission error and to optimize a data rate.

In addition, this standard may use a coding scheme which transmitsseveral duplicated copies to increase data reliability and also may usethe OFDM to support a higher data rate.

With the widespread use of the WLAN and the diversification ofapplications using the WLAN, there is a recent demand for a new WLANsystem to support a higher throughput than a data processing speedsupported by the IEEE 802.11n. A very high throughput (VHT) WLAN systemis one of IEEE 802.11 WLAN systems which have recently been proposed tosupport a data processing speed of 1 Gbps or more. The VHT WLAN systemis named arbitrarily. To provide a throughput of 1 Gbps or more, afeasibility test is currently being conducted for the VHT system whichuses MIMO and a channel bandwidth of 80 MHz or more and which also usesa spatial division multiple access (SDMA) scheme as a channel accessscheme.

The conventional channel access mechanism used in the IEEE 802.11n WLANsystem or other WLAN systems cannot be directly used as a channel accessmechanism of a WLAN system for providing a throughput of 1 Gbps or more(hereinafter, such a WLAN system is referred to as a VHT WLAN system).This is because a channel bandwidth used by the VHT WLAN system is atleast 80 MHz since the conventional WLAN system operates under thepremise of using a channel bandwidth of 20 MHz or 40 MHz which is toonarrow to achieve the throughput of 1 Gbps or more in a service accesspoint (SAP).

Therefore, in order for a VHT basic service set (BSS) to satisfy a totalthroughput of 1 Gbps or more, several VHT STAs need to simultaneouslyuse a channel in an effective manner. A VHT AP uses SDMA to allow theseveral VHT STAs to simultaneously use the channel in an effectivemanner. That is, the several VHT STAs are allowed to simultaneouslytransmit and receive data to and from the VHT AP.

A modulation and coding scheme (MCS) feedback method is one of methodsfor more effectively supporting link adaptation in such an IEEE 802.11nMIMO environment. A link adaptation procedure uses a specific MCS withgiven link quality to increase a data throughput by using a highesttransfer rate. However, since the conventional MCS feedback method isachieved under the premise that one-to-one communication is achievedbetween a station and an AP, the conventional method needs to becompensated when it applies to a multi-user MIMO environment.

SUMMARY OF THE INVENTION

In a link adaptation protocol, a modulation and coding scheme (MCS)feedback procedure is limited to be used only for a point-to-pointtransmission scenario. Therefore, there is a problem in that anenvironment such as a multi-user multiple input multiple output (MIMO)environment cannot sufficiently consider other factors which may occurin point-to-multi point transmission.

A link adaptation scheme suitable for the multi-user environment isprovided according to embodiments of the present invention. Influencecaused by other users can be taken into consideration when several userssimultaneously perform data transmission or reception. In this case, anactual communication environment can be taken into consideration toperform link adaptation with more accurate information.

In an aspect of the present invention, a method of performing a linkadaptation procedure for multi-user transmission in a wireless localarea network (WLAN) system includes receiving a modulation and codingscheme (MCS) feedback request transmitted by an access point (AP) to aplurality of stations, estimating an MCS by considering a spatial streamcorresponding to the MCS feedback request transmitted to the otherstations, and transmitting an MCS feedback response comprising theestimated MCS to the AP.

The MCS feedback request may be transmitted by being included in asounding physical layer convergence procedure (PLCP) protocol data unit(PPDU) steered in accordance with a pre-coding vector corresponding toeach station receiving the MCS feedback request.

The MCS feedback request may comprise an MCS feedback order fordesignating an order of each MCS feedback response corresponding to theMCS feedback request, and the MCS feedback response may be transmittedat a time point depending on the MCS feedback order.

The MCS feedback request may be received together with a null datapacket (NDP) announcement, and the method may further comprise receivingan NDP frame from the AP immediately after receiving the NDPannouncement and the MCS feedback request.

The MCS may be estimated by using the NDP frame.

Influence caused by other users can be taken into consideration whenseveral users simultaneously perform data transmission or reception.Further, an actual communication environment can be taken intoconsideration to perform link adaptation with more accurate information.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing an exemplary structure of a very highthroughput (VHT) wireless local area network (WLAN) system according toan embodiment of the present invention.

FIG. 2 is a flowchart showing an example of a link adaptation schemeaccording to the prior art.

FIG. 3 shows a problem of a link adaptation scheme according to theprior art.

FIG. 4 is a flowchart showing an example of a method of performing alink adaptation procedure according to an embodiment of the presentinvention.

FIG. 5 shows a link adaptation procedure according to another embodimentof the present invention.

FIG. 6 shows a link adaptation procedure according to another embodimentof the present invention.

FIG. 7 shows a link adaptation procedure according to another embodimentof the present invention.

FIG. 8 is a block diagram of a wireless communication apparatus forperforming a link adaptation procedure according to an embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic view showing an exemplary structure of a very highthroughput (VHT) wireless local area network (WLAN) system according toan embodiment of the present invention.

Referring to FIG. 1, a WLAN system such as the VHT WLAN system includesone or more basis service sets (BSSs). The BSS is a set of stations(STAs) which are successfully synchronized to communicate with oneanother, and is not a concept indicating a specific region. As in theWLAN system to which the embodiment of the present invention isapplicable, a BSS that supports a super high-speed data processing of 1GHz or more is referred to as a VHT BSS.

The VHT BSS can be classified into an infrastructure BSS and anindependent BSS (IBSS). The infrastructure BSS is shown in FIG. 1.

Infrastructure BSSs (i.e., BSS1 and BSS2) include one or more non-accesspoint (AP) STAs (i.e., Non-AP STA1, Non-AP STA3, and Non-AP STA4), APSTAs (i.e., AP STA1 and AP STA2) which are STAs providing a distributionservice, and a distribution system (DS) connecting the plurality of APSTAs (i.e., AP STA1 and AP STA2). In the infrastructure BSS, an AP STAmanages non-AP STAs of the BSS.

On the other hand, the IBSS is a BSS operating in an ad-hoc mode. Sincethe IBSS does not include the VHT STA, a centralized management entityfor performing a management function in a centralized manner does notexist. That is, the IBSS manages the non-AP STAs in a distributedmanner. In addition, in the IBSS, all STAs may consist of mobile STAs,and a self-contained network is configured since connection to the DS isnot allowed.

The STA is an arbitrary functional medium including a medium accesscontrol (MAC) and wireless-medium physical layer (PHY) interfaceconforming to the institute of electrical and electronics engineers(IEEE) 802.11 standard, and includes both an AP and a non-AP STA in abroad sense. A VHT STA is defined as an STA that supports the superhigh-speed data processing of 1 GHz or more in the multi-channelenvironment to be described below. In the VHT WLAN system to which theembodiment of the present invention is applicable, STAs included in theBSS may be all VHT STAs, or a VHT STA and a legacy STA (i.e., IEEE802.11n-based HT STA) may coexist.

The STA for wireless communication includes a processor and atransceiver, and also includes a user interface, a display means, etc.The processor is a functional unit devised to generate a frame to betransmitted through a wireless network or to process a frame receivedthrough the wireless network, and performs various functions to controlSTAs. The transceiver is functionally connected to the processor and isa functional unit devised to transmit and receive a frame for the STAsthrough the wireless network.

Among the STAs, non-AP STAs (i.e., STA1, STA3, STA4, and STA5) areportable terminals operated by users. A non-AP STA may be simplyreferred to as an STA. The non-AP STA may also be referred to as aterminal, a wireless transmit/receive unit (WTRU), a user equipment(UE), a mobile station (MS), a mobile terminal, a mobile subscriberunit, etc. A non-AP VHT-STA (or simply VHT STA) is defined as a non-APSTA that supports the super high-speed data processing of 1 GHz or morein the multi-channel environment to be described below.

The AP (i.e., AP1 and AP2) is a functional entity for providingconnection to the DS through a wireless medium for an associated STA.Although communication between non-AP STAs in an infrastructure BSSincluding the AP is performed via the AP in principle, the non-AP STAscan perform direct communication when a direct link is set up. Inaddition to the terminology of an access point, the AP may also bereferred to as a centralized controller, a base station (BS), a node-B,a base transceiver system (BTS), a site controller, etc. A VHT AP isdefined as an AP that supports the super high-speed data processing of 1GHz or more in the multi-channel environment to be described below.

A plurality of infrastructure BSSs can be interconnected by the use ofthe DS. An extended service set (ESS) is a plurality of BSSs connectedby the use of the DS. STAs included in the ESS can communicate with oneanother. In the same ESS, a non-AP STA can move from one BSS to anotherBSS while performing seamless communication.

The DS is a mechanism whereby one AP communicates with another AP. Byusing the DS, an AP may transmit a frame for STAs associated with a BSSmanaged by the AP, or transmit a frame when any one of the STAs moves toanother BSS, or transmit a frame to an external network such as a wirednetwork. The DS is not necessarily a network, and has no limitation inits format as long as a specific distribution service specified in theIEEE 802.11 can be provided. For example, the DS may be a wirelessnetwork such as a mesh network, or may be a physical construction forinterconnecting APs.

FIG. 2 is a flowchart showing an example of a link adaptation schemeaccording to the prior art. The link adaptation scheme of FIG. 2 is forsingle-user MIMO.

The link adaptation scheme is performed by exchanging modulation andcoding scheme (MCS) information between an MCS feedback requester andits counterpart terminal. Herein, an AP corresponds to the MCS feedbackrequester, and an MCS feedback responder is a user terminal, which isreferred in the present invention as a station (STA).

To receive an MCS feedback from the STA, the AP transmits an MCSfeedback request (MRQ) to the STA (step S210). The MRQ can betransmitted using a link adaptation control subfield of a highthroughput control (HTC) field conforming to the IEEE 802.11n standard.In addition, in order to allow an STA receiving the MRQ to perform MCScalculation, the AP can transmit the MRQ by using a sounding physicallayer convergence procedure (PLCP) protocol data unit (PPDU).

The STA estimates an MCS value (step S220). Further, in response to theMRQ, an MCS feedback response is transmitted, and the estimated MCSvalue is transmitted to the AP (step S230). The estimated MCS value canalso be transmitted to the AP by using the link adaptation controlsubfield of the HTC field.

FIG. 3 shows a problem of a link adaptation scheme according to theprior art.

An AP performs downlink transmission simultaneously to several STAs inmulti-user MIMO. Alternatively, a plurality of STAs perform uplinktransmission to the AP.

Referring to FIG. 3, an STA 1 and an STA 2 can perform uplinktransmission and downlink transmission, and the two STAs cansimultaneously perform uplink transmission or downlink transmission. Itis assumed that the AP has two or more antennas, and each STA has oneantenna.

If the STA 1 and the STA 2 simultaneously perform uplink transmission(steps 310 and 330), uplink transmission (step 310) of the STA 1 may actas interference to uplink transmission (step 330) of the STA 2, andlikewise, uplink transmission (step 330) of the STA 2 may act asinterference to uplink transmission (step 310) of the STA 1.

The same also apply to a downlink scenario. That is, when the APperforms downlink transmission simultaneously to the STA 1 and the STA 2(steps 320 and 340), downlink transmission (step 320) to the STA 1 mayact as interference to downlink transmission (step 340) of the STA 2,and likewise, downlink transmission (step 340) to the STA 2 may act asinterference to downlink transmission (step 320) of the STA 1.

Therefore, for a link adaptation protocol in multi-user MIMO,interference generated by a spatial stream to be transmitted to anotherSTA needs to be taken into consideration when performing MCS estimation.

That is, when the AP performs downlink transmission simultaneously tothe STAs, each STA is interfered by a spatial stream to be transmittedtoward another STA. Even if the AP performs simultaneous transmission byselecting STAs having a low channel correlation, constraint orcancellation of interference cannot be guaranteed.

FIG. 4 is a flowchart showing an example of a method of performing alink adaptation procedure according to an embodiment of the presentinvention.

A link adaptation protocol in multi-user MIMO is proposed in theembodiment of the present invention. Although a downlink transmissionscenario is described in this embodiment described with reference toFIG. 4, a link adaptation protocol according to the embodiment of thepresent invention may also equally apply to uplink transmission.

For example, when downlink transmission is performed simultaneously to aplurality of STAs, interference between STAs can be minimized in such amanner that an AP transmits streams by selecting and grouping STAshaving a low channel correlation if downlink transmissions aresimultaneously performed, thereby facilitating interference reduction.However, as described above, an MCS value estimated in single-user MIMOis problematic in a sense that interference generated by a spatialstream to be transmitted toward other terminals cannot be properlyconsidered in a multi-user MIMO environment. In addition, when usingonly a method of transmitting data by classifying the STAs according toa channel correlation, interference cannot be effectively reduced orconsidered in the link adaptation procedure, and an MCS cannot beaccurately recognized.

In order to allow accurate MCS estimation when an STA estimates an MCSand feeds back the MCS to an AP, the AP can transmit an MCS feedbackrequest (MRQ) simultaneously to STAs for performing downlinktransmission by using a sounding PPDU. More specifically, the APtransmits the MRQ to an STA 1 and an STA 2 by using a sounding PPDU(steps S410 and S420). In doing so, the respective STAs can moreeffectively estimate an MCS at that moment on a real time basis byconsidering an interference level caused by spatial streams to betransmitted toward other STAs.

In FIG. 4, an example of downlink multi-user MIMO is shown. A linkadaptation scheme of the present invention can also equally apply touplink multi-user MIMO.

The STAs (i.e., STA 1 and STA 2) receiving the MRQ from the AP estimatean MCS by considering mutual interference. That is, the STA 1 estimatesan MCS value of the STA 1 by considering a spatial stream transmitted tothe STA 2, and the STA 2 estimates an MCS value of the STA 2 byconsidering a spatial stream transmitted to the STA 1.

Further, the STA 1 and the STA 2 transmit an MCS feedback response tothe AP, and the MCS feedback response includes information on the MCSvalues estimated respectively by the STA 1 and the STA 2 (steps S450 andS460). By using the MCS values estimated by this process inconsideration of a communication environment such as currently generatedinterference, a link throughput can be more increased by performing alink adaptation scheme suitable for a multi-user environment.

FIG. 5 shows a link adaptation procedure according to another embodimentof the present invention. Similarly to FIG. 4, uplink transmission isachieved in the embodiment of FIG. 5. However, it is obvious that a linkadaptation method according the embodiment of the present invention canalso equally apply to uplink transmission.

The embodiment of FIG. 5 is similar to an actual wireless communicationenvironment in a sense that STAs of a more number of users participatein transmission, and in particular, is different from the embodiment ofFIG. 4 in a sense that an order is assigned to an MCS feedback.

In this case, an AP is an MCS feedback requester. STAs (i.e., STA 1, STA2, STA 3, and STA 4) feedback an MCS to the AP, and the AP intends toperform downlink multi-user transmission to the STA 1, the STA 2, theSTA 3, and the STA 4.

The AP transmits an MCS feedback request (MRQ) simultaneously to the STA1, the STA 2, the STA 3, and the STA 4 (step S510). In this case, theMRQ is transmitted using a sounding PPDU, and is transmitted by beingsteered in accordance with a pre-coding vector configured for eachreceiving terminal.

Each of the STAs (i.e., STA 1, STA 2, STA 3, and STA 4) estimates an MCSby using the received sounding PPDU. In particular, since the AP hastransmitted the sounding PPDU simultaneously to the STA 1, the STA 2,the STA 3, and the STA 4, each STA can performs MCS estimation byconsidering interference generated by a spatial stream corresponding tothe sounding PPDU transmitted to another STA.

An MCS feedback sequence identifier may be included in the MRQtransmitted by the AP to the STAs. However, when the AP transmits theMRQ simultaneously to the respective STAs, the MCS feedback sequenceidentifier is set to the same value. This implies that an MCS feedbackto be responded by the STAs corresponds to the same MCS feedbackrequest.

In this case, the MCS feedback requester sets the same MCS feedbacksequence identifier to STAs for performing multi-user transmission so asto effectively manage MCS feedback reports and so as to decrease wasteof identifier space for the MCS feedback sequence identifier. The MCSfeedback sequence identifier may be included in a link adaptationcontrol subfield.

If the MRQ has the same MCS feedback sequence identifier, an MCSfeedback sequence identifier of an MCS feedback transmitted by each STAalso has the same value. Therefore, when an AP (i.e., an MCS feedbackrequester) receives a plurality of MCS feedbacks having the same MCSfeedback sequence identifier, it can be seen that these values are MCSvalues estimated by the respective STAs corresponding to multi-usertransmissions simultaneously performed.

The MRQ may include an MCS feedback order. The MRQ can assigntransmission orders of MCS feedbacks transmitted by a plurality of STAs(i.e., STA 1, STA 2, STA 3, and STA 4) to an AP. Therefore, the STA 1,the STA 2, the STA 3, and the STA 4 transmit MCS feedback responsessequentially according to the MCS feedback orders included in thereceived MRQs (step S520, S530, S540, and S550). In the embodimentdescribed with reference to FIG. 5, it is assumed that the MCS feedbackorders are set such that the MCS feedback responses are transmittedaccording to the orders of the STA 1, the STA 2, the STA 3, and the STA4. By setting the MCS feedback orders, the MCS feedbacks can beprevented from collision.

In a case where the AP transmits the MRQ while intending to receive anMCS feedback response for that request at a specific time later, the APmay set a delayed time and broadcast the MRQ after the delayed time byusing a frame different from that used for sounding PPDU transmission.In this case, the MRQ may include information regarding an orderaccording to which a corresponding MCS feedback is transmitted.

In addition, the MCS feedback sequence identifier and the MCS feedbackorder included in the MRQ may also equally apply to other embodiments ofthe present invention.

FIG. 6 shows a link adaptation procedure according to another embodimentof the present invention.

A link adaptation protocol shown in FIG. 6 uses a null data packet (NDP)in multi-user MIMO.

Among MAC data types conforming to the IEEE 802.11 standard, a null dataframe implies that only a MAC header exists whereas a MAC service dataunit (MSDU) does not exist. On the other hand, the NDP implies that onlya PHY header exists whereas a physical layer convergence procedure(PLCP) service data unit (PSDU) does not exist as well as actual data.

Since the NDP does not have a MAC header, there is no field indicating asource address, a destination address, etc. Therefore, to transmit theNDP, a non-NDP PPDU must be transmitted prior to transmission of theNDP. The NDP is transmitted immediately after the non-NDP PPDU istransmitted. The non-NDP PPDU implies a normal PPDU other than the NDP.An NDP announcement must be set in the NDP PPDU so as to announce to areceiving side that the NDP will be transmitted soon.

In the 802.11n standard, the NDP is announced by setting a bitcorresponding to an NDP announcement of an HTC field to 1. A sourceaddress and a destination address of a non-NDP PPDU that is an NDPannouncement frame are a source address and a destination address of anNDP. The NDP is a sounding PPDU, and the receiving side that receivesthe NDP performs channel estimation by using the NDP.

Also in this case, an AP is an MCS feedback requester and intends toperform downlink multi-user transmission to an STA 1, an STA 2, an STA3, and an STA 4. As described above, the link adaptation procedureaccording to the embodiment of the present invention can also apply notonly to downlink transmission but also to uplink transmission.

The AP transmits an MCS feedback request (MRQ) simultaneously to the STA1, the STA 2, the STA 3, and the STA 4, and sets an NDP announcement ina PPDU including the MRQ (step S610). Subsequent to transmission of theMRQ, NDP frames are respectively transmitted to the STA 1, the STA 2,the STA 3, and the STA 4 (step S620). In this case, the NDP frames aretransmitted by being steered in accordance with a pre-coding vectorconfigured for each STA.

The STA 1, the STA 2, the STA 3, and the STA 4 may estimate an MCS byusing the received NDP frames, and may transmit an MCS feedback responseto the AP in response to the previously received MRQ (steps S630, S640,S650, and S660). Of course, the MCS feedback response includes anestimated MCS value. In addition, since the AP transmits an NDP framesimultaneously to the STA 1, the STA 2, the STA 3, and the STA 4, eachSTA can perform MCS estimation by considering interference caused by aspatial stream corresponding to the NDP frame to be transmitted toanother STA.

FIG. 7 shows a link adaptation procedure according to another embodimentof the present invention.

According to the embodiment of FIG. 7, a link training procedure and alink adaptation procedure are simultaneously performed.

In case of the link training, an AP transmits a training request message(TRM) to STAs (i.e., STA 1, STA 2, STA 3, and STA 4) (step S710). TheTRM is transmitted simultaneously to the respective STAs by beingincluded in a sounding PPDU steered in accordance with a pre-codingvector configured for each STA. An MCS feedback request (MRQ) isincluded also in the sounding PPDU transmitted by the AP to the STAs.

Upon receiving the TRM and the steered sounding PPDU including the MRQ,the STAs (i.e., STA 1, STA 2, STA 3, and STA 4) transmits an unsteeredsounding PPDU to an AP STA so that the AP can perform channel estimation(steps S720, S730, S740, and S750). It is assumed herein that an uplinkchannel and a downlink channel have mutually reversible characteristics.Therefore, the present embodiment can also apply to both uplinktransmission and downlink transmission.

That is, when the link training and the link adaptation are performedsimultaneously, the AP transmits the TRM by using the sounding PPDU, andat the same time, transmits the MRQ. In this case, the sounding PPDU isa steered PPDU to be transmitted toward each STA. Further, uponreceiving the sounding PPDU transmitted from the AP, the STAs estimatean MCS by further considering spatial steams corresponding to soundingPPDUs transmitted from other STAs, and thereafter respond to the AP bytransmitting an MCS feedback.

In this case, for the link training requested by the AP by using theTRM, the STAs (i.e., STA 1, STA 2, STA 3, and STA 4) transmit MCSfeedbacks also by using the sounding PPDU. The sounding PPDU transmittedby the STA 1, the STA 2, the STA 3, and the STA 4 is for channelestimation of the AP, and is transmitted in a state of an unsteeredPPDU.

The AP performs channel estimation by using the sounding PPDUtransmitted by the STA 1, the STA 2, the STA 3, and the STA 4. If thereis a change in a channel state between the AP and any one of the STA 1,the STA 2, the STA 3, and the STA 4, the AP uses the changed channelstate to correct an MCS value responded by a corresponding STA.Otherwise, if there is no change in the channel state between the AP andany one of the STA 1, the STA 2, the STA 3, and the STA 4, the AP usesthe MCS value responded by the STA 1, the STA 2, the STA 3, and the STA4 to perform uplink or downlink multi-user transmission.

FIG. 8 is a block diagram of a wireless communication apparatus forperforming a link adaptation procedure according to an embodiment of thepresent invention. The aforementioned STAs may be an example of thewireless communication apparatus of FIG. 8.

The wireless communication apparatus includes a processor 810 and aradio frequency (RF) unit 820. A memory 830 is coupled to the processor810 and stores a variety of information to drive the processor 810. Thememory 830 may include a read-only memory (ROM), a random access memory(RAM), a flash memory, a memory card, a storage medium, and/or otherequivalent storage devices. In addition thereto, the wirelesscommunication apparatus may further include a display unit or a userinterface. Since this is apparent to those ordinary skilled in the art,the display unit or the user interface is not depicted in FIG. 8, anddetailed descriptions thereof will be omitted.

The wireless communication apparatus described with reference to FIG. 8can perform the link adaptation procedure or the method of performingthe link adaptation procedure according to the embodiments of thepresent invention described above with reference to FIG. 1 to FIG. 7.

The processor 810 may include an application-specific integrated circuit(ASIC), a separate chipset, a logic circuit, and/or a data processingunit. The processor 810 generates a control signal or data to betransmitted to another STA or AP. The processor 810 estimates an MCSvalue upon receiving an MCS feedback request from an AP via the RF unit820.

The processor 810 also considers interference caused by a spatial streamcorresponding to an MCS feedback request transmitted to another terminalin the process of MCS estimation. Therefore, the MCS can be moreaccurately estimated by considering a communication environment on areal time basis. Further, the processor 810 generates an MCS feedbackresponse including the estimated MCS.

The RF unit 820 is coupled to the processor 810. The RF unit 820transmits a radio signal generated by the processor 810, and receives aradio signal transmitted by another wireless communication apparatus.The RF unit 820 may include a baseband circuit for processing the radiosignal. Signals can be transmitted in a broadcast or unicast manner.According to the embodiment of the present invention, the RF unit 820can receive an MCS feedback request and/or a training request messagefrom the AP, and can transmit an MCS feedback response generated by theprocessor 810 to the AP.

All functions described above may be performed by a processor such as amicroprocessor, a controller, a microcontroller, an application specificintegrated circuit (ASIC), or a processor of a terminal (e.g., thewireless communication apparatus illustrated in FIG. 8) according tosoftware or program code for performing the functions. The program codemay be designed, developed, and implemented on the basis of thedescriptions of the present invention, and this is well known to thoseskilled in the art.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention as defined by the appended claims. The exemplary embodimentsshould be considered in descriptive sense only and not for purposes oflimitation. Therefore, the scope of the invention is defined not by thedetailed description of the invention but by the appended claims, andall differences within the scope will be construed as being included inthe present invention.

What is claimed is:
 1. A method of performing link adaptation in awireless local area network system by a mobile station, the methodcomprising: receiving, by the mobile station, a first physical layerprotocol data unit (PPDU) transmitted by an access point (AP) viamulti-user multiple input/multiple output (MU MIMO) transmission to aplurality of mobile stations, the first PPDU including a first feedbacksequence identifier that includes a value identifying the plurality ofmobile stations, when the mobile station is one of the plurality ofmobile stations identified by the value included by the first feedbacksequence identifier, estimating a modulation and coding scheme (MCS)based on the first PPDU; and transmitting, by the mobile station, asecond PPDU to the AP, wherein the second PPDU includes channelinformation about the estimated MCS and a second feedback sequenceidentifier, the second feedback sequence identifier including the valueincluded by the first feedback sequence identifier.
 2. The method ofclaim 1, wherein the first PPDU further includes an MCS feedback requesttransmitted by the AP to the plurality of mobile stations.
 3. The methodof claim 1, wherein the first PPDU includes a feedback order thatindicates a first mobile station of the plurality of mobile stations tosend a response for the first PPDU.
 4. A device comprising: a radiofrequency unit configured to receive and transmit radio signals; and aprocessor operatively coupled with the radio frequency unit andconfigured to: instruct the radio frequency unit to receive a firstphysical layer protocol data unit (PPDU) transmitted by an access point(AP) via multi-user multiple input/multiple output (MU MIMO)transmission to a plurality of mobile stations, the first PPDU includinga first feedback sequence identifier that includes a value identifyingthe plurality of stations, estimate a modulation and coding scheme (MCS)based on the first PPDU when the device is one of the plurality ofstations identified by the value included by the first feedback sequenceidentifier, and instruct the radio frequency unit to transmit a secondPPDU to the AP, wherein the second PPDU includes channel informationabout the estimated MCS and a second feedback sequence identifier, thesecond feedback sequence identifier including the value included by thefirst feedback sequence identifier.
 5. The device of claim 4, whereinthe first PPDU further includes an MCS feedback request transmitted bythe AP to the plurality of mobile stations.
 6. The device of claim 4,wherein the first PPDU includes a feedback order that indicates a firstmobile station of the plurality of mobile stations to send a responsefor the first PPDU.